Laminated ceramic capacitors produced by simultaneous sintering of dielectric ceramic composition and internal electrodes have been developed in recent years as compact, large-capacity capacitor elements. Traditional dielectric ceramic compositions have high sintering temperatures of 1150° C. to 1400° C. and therefore nickel (Ni) or nickel alloy that withstands high temperatures has been used as the leading electrode material for simultaneous sintering with internal electrodes. However, nickel is a rare metal and as the demand for rare metals is expected to grow and prices of rare metals escalate in recent years, alternative technologies are drawing attention, and there is a growing demand for replacing internal electrodes using nickel (hereinafter also referred to as “Ni”) with internal electrodes using copper (hereinafter also referred to as “Cu”) or other metal whose bullion is cheaper.
However, since copper has a melting point of 1085° C., which is lower than the melting point of nickel, use of copper in internal electrodes requires sintering to be implemented at 1030° C. or preferably 1000° C. or below, which gives rise to a problem, or specifically a need for dielectric material for laminated ceramic capacitors that can demonstrate sufficient characteristics even when sintered at temperatures lower than the temperatures traditionally used.
In light of the aforementioned situations, the inventors of the present invention studied with the purpose of obtaining a laminated ceramic capacitor which can be sintered at 1030° C. or preferably 1000° C. or below in a reducing ambience, which does not contain lead (Pb) or bismuth (Bi) in its dielectric ceramic layers, and which has a dielectric constant of 2000 or more, X7R temperature characteristics of dielectric constant and high-temperature stress longevity traits equivalent to conventional laminated ceramic capacitors with Ni internal electrodes, and discovered conditions for Ba/Ti ratio, composition ratio of rare earths as auxiliary components, and MnO composition ratio, for a dielectric ceramic composition whose primary component is a BaTiO3 compound. Based on the above, the inventors of the present invention propose a laminated ceramic capacitor having: multiple dielectric ceramic layers; internal electrodes which are formed between the dielectric ceramic layers in a manner opposing each other and led out alternately to different end faces; and external electrodes which are formed on both end faces of the dielectric ceramic layers and connected electrically to the internal electrodes; wherein such laminated ceramic capacitor is characterized in that: the dielectric ceramic layer is a sintered compact constituted by a primary component expressed by ABO3+aRe2O3+bMnO (where ABO3 is a perovskite dielectric mainly constituted by BaTiO3, Re2O3 represents at least one type of metal oxide selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and a and b represent mol numbers relative to 100 mol of ABO3) in the ranges of 1.000≦A/B≦1.035, 0.05≦a≦0.75, and 0.25≦b≦2.0, as well as auxiliary components that include at least one type of element selected from B, Li, and Si for a total of 0.16 to 1.6 parts by mass in equivalent B2O3, Li2O, and SiO2, respectively; and that the internal electrodes are constituted by Cu or Cu alloy (Patent Literature 1).
The inventors of the present invention also proposed that, with a laminated ceramic capacitor whose internal electrodes are constituted by Cu or Cu alloy, X7R or X8R temperature characteristics can be achieved by obtaining its dielectric ceramic as a sintered compact of perovskite dielectric material primarily constituted by BaTiO3, comprised of grains whose average diameter is 400 nm or less in a section view as well as grain boundaries (Patent Literature 2), where examples illustrate mixtures of MnO as a starting material for sintered compact, with B2O3, Li2O, and SiO2, as additive rare earth oxides and sintering auxiliaries.
Furthermore, the inventors of the present invention discovered that, with a dielectric ceramic composition constituted by a primary component constituted by BaTiO3 as well as auxiliary components constituted by Re, Mn, V, Mo, Cu, B, Li, Ca, and Sr, the total content of V and Mo, in addition to the contents of Re, Mn, B, and Li, would affect the life characteristics of the laminated ceramic capacitor using internal electrodes whose primary component is Cu, and proposed a dielectric ceramic composition characterized in that it is expressed by BaTiO3+aRe2O3+bMnO+cV2O5+dMoO3+eCuO+fB2O3+gLi2O+xSrO+yCaO (where Re represents at least one type of element selected from Eu, Gd, Dy, Ho, Er, Yb, and Y, and a to h represent mol numbers relative to 100 mol of the primary component constituted by BaTiO3) and, when the mol ratio of (Ba+Sr+Ca)/Ti contained in the dielectric ceramic composition is given by m, 0.10≦a≦0.50, 0.20≦b≦0.80, 0≦c≦0.12, 0≦d≦0.07, 0.04≦c+d≦0.12, 0≦e≦1.00, 0.50≦f≦2.00, 0.6≦(100 (m−1)+2g)/2f≦1.3, 0.5≦100 (m−1)/2g≦5.1, 0≦x≦1.5, and 0≦y≦1.5 are satisfied (Patent Literature 3).
In the meantime, thinning of dielectric ceramic layers constituting laminated ceramic capacitors is being examined as one effective means for meeting the required size reduction and capacity increase, and various proposals have been presented regarding design for reliability after thinning For example, Patent Literature 4 describes mixing crushed material powder and uncrushed material powder at an appropriate ratio to obtain a structure where one peak appears in an area of less than one-quarter of the layer thickness and another in an area of more than that, in order to achieve both high dielectric constant and high electrical insulation property even when the layer is made as thin as less than 1 μm. In addition, Patent Literature 5 adopts a structural method whereby the number of grains per layer is reduced and limited to facilitate diffusion of Mn and V across the grain boundaries, to permit layer thickness reduction and achieve good longevity traits. Furthermore, Patent Literature 6 achieves both layer thickness reduction to less than 1 μm and good longevity traits at the same time by containing a Li compound as a sintering auxiliary and adjusting the average grain size Rg [μm] of the dielectric ceramic to 0.06<Rg<0.17 and its standard deviation σg [μm] to σg<0.75, while Patent Literature 7 adopts a method whereby a compound having the same composition as the secondary phase is mixed into the dielectric material before sintering, to suppress excessive generation of the secondary phase, so that generation of the secondary phase, which can cause the reliability to drop, can be limited. With this method, the size of the secondary phase is limited to one-third or less of the thickness of the dielectric. In addition, Patent Literature 8 describes that temperature characteristics of capacitance, dielectric breakdown voltage, and high-temperature load longevity could be improved with a thin-layer structure by separating the M4R6O(SiO4)6 crystal phase (M represents at least one type of alkali earth metal) at the grain boundary layers sandwiched between two primary phase grains constituted by BaTiO3, as well as at the triple points. Furthermore in Patent Literature 9, it was found that separation of excessive Ba, Si, and other alkali earth metals at the triple points as noncrystalline substances was lowering the reliability. Accordingly, at least 80% of the cross-section area at triple points was kept to 8 nm or below, and crystalline oxide grains containing Ba, Ti, and Si were separated into the dielectric ceramic layers instead, to achieve high reliability with this thin-layer structure.